What is View Mail Statistics

I have no idea what this menu is under WHM. I see graphs and numbers but

(1) I am not sure what each title means specifically

(2) I think I know what the title means but not sure I know what I'm seeing, e.g. "Top 50 local senders by message count" I assume are messages sent by account but what is mailnull? I see "Top 50 sending hosts by volume" but I also see my server listed once as well so are those bounces? Emails from one domain to another via my server from one domain to another... I also see "local" in this list - does that mean stuff sent using my server via a script for example? Recently I'm seeing a lot of activity on my server and can't be sure if one of the accounts on my server is sending email so how does one know for sure which account is doing the sending? I think that would be even more useful than other things here.

Anyone find a write up on the details? The CPANEL manual is woefully sparse.